Airborne measurements of forest and agricultural land surface emissivity at millimeter wavelengths

Passive microwave radiometers have been operated on an aircraft over the an area of Sweden, near Uppsala in September 1995 and March 1997 as part of the Northern Processes Experiment (NOPEX). Their measurements have allowed the calculation of the emissivity of boreal forest and agricultural land surfaces at 24, 50, 89, and 157 GHz over a range of incidence angles and polarizations. These results show consistent differences between dense forestry, where the emissivity is close to 1 and open land, where it is approximately 0.96. These differences are examined and a model is presented to parameterize these surfaces by use of a Debye-like effective permittivity and Fresnel's reflection coefficients. This will allow retrievals of atmospheric temperature and humidity profiles to be made by satellite microwave sounders, such as the advanced microwave sounding unit (AMSU) over similar land surfaces.